Your story is the exact same reason I got my fourth (Brody). I had Tiffany and Tyler 7 and 8 years old and then I brought Chloe. Well they had their own bond going and they never did play much with her although she was very demanding of them.
So I convinced hubby and we got Brody. Brody came to us when Chloe had just turned one. They have the most incredible bond and they love to play and sleep together. It was the best decision we ever made.
Now guess what? The older ones at times (not always) play with Brody and Chloe. Its really funny to see the four of them running around the house playing "tag your it".
Actually potty training was much easier, Brody picked it up quickly!
Congrats to you
))